1. Field of the Invention
The invention relates to materials and methods for fabricating thin film electrical components in integrated circuits and, more particularly, liquid precursor solutions which are used in misted deposition apparatus for depositing thin films.
2. Description of the Related Art
As is well-known in the art, the electrical components in integrated circuits are made up of layers of thin films which are connected by wiring layers and separated by insulating layers. Simple thin film materials and compounds, such as silicon glass, have been formed using a liquid deposition process. Complex compounds, i.e., compounds containing more than two elements, in the prior art have always been formed using processes such as vacuum sputtering (i.e., E-beam, D.C., R.F., ion-beam, etc.), laser ablation, reactive chemical vapor deposition including metalorganic chemical vapor deposition (MOCVD); and liquid application methods using sol-gels (alkoxides) or carboxylates. However, none of these known methods have been able to produce metal oxides with properties that are entirely satisfactory for use in integrated circuits. In all of the prior art processes, except sputtering, the films produced had significant physical defects, such as cracking, peeling, etc.
It was substantially impossible with the conventional processes, particularly sputtering, to reliably and repeatably produce metal oxides with a specific stoichiometry within tolerances required for integrated circuits. Some processes, like MOCVD, could be dangerous or toxic. Most required high temperatures that were destructive to an integrated circuit, and provided poor xe2x80x9cstep coveragexe2x80x9d of a substrate to be covered, e.g., the prior art techniques resulted in a relatively excessive build-up of deposition of the film at the boundary of any discontinuities on the substrate. In prior art liquid deposition processes, it was impossible to control thickness with the degree of accuracy that is required to manufacture integrated circuits. As a result, up to now, metal oxides and other complex materials have not been used in integrated circuits except for one or two specialty, relatively expensive applications, such as the use of sputtered lead zirconium titanate (PZT) in ferroelectric integrated circuits that were expected to have short life times.
U.S. Pat. No. 5,456,945 to McMillan et al. provided a substantial advance in the art by teaching the use of ultrasonic transducers to generate a volume of mist or aerosol into a deposition chamber. Within the deposition chamber, a DC voltage is applied between a substrate holder and a barrier plate for induced polarization in the aerosol particles. A constant flow of carrier gas, e.g., argon, is used to transport the aerosol to an integrated circuit substrate or wafer for the deposition of liquid thin films. These liquid thin films are dried and annealed to yield the thin films of an integrated circuit.
When the precursor is processed in misted deposition apparatus according to the U.S. Pat. No. 5,456,945, the selection or choice of a liquid precursor affects step coverage, liquid film deposition rate, and the final thin film morphology. The use of some liquid precursor solutions results in no material being deposited as desired. Other solutions provide poor step coverage, and yet other solutions have good step coverage with poor film morphology. There is a need for liquid precursor solutions that function in misted deposition apparatus to provide good step coverage and good film morphology. There is also a need for parameters permitting the design and development of precursor solutions that will function appropriately in misted deposition apparatus.
The claimed invention overcomes the problems outlined above to advance the art by providing a cosolvent system for use in liquid precursor solutions for misted deposition apparatus. The liquid precursors are processed in misted deposition apparatus to yield metal oxide thin films having good step coverage and good film morphology rendering the metal oxide thin films suitable for use in integrated circuits. Additionally, the cosolvent system enhances the film deposition rate.
Method and apparatus according to the invention utilize a liquid precursor solution to form metal compounds. The solution is processed in a misted deposition apparatus to yield films having good step coverage and good morphology. The liquid precursor comprises a metal organic portion including at least one metal organic compound. The metal organic portion has a total metal content in an effective amount for yielding a solid metal compound during an anneal of a thin film of the liquid precursor solution. For example, the metal organic portion may contain barium, strontium, and titanium metals to yield barium strontium titanate (BST) in an oxygen anneal. The resultant BST has a stoichiometry corresponding to that of the liquid precursor solution for these metals less any volatilization losses of metals during the anneal.
The metal organic portion is mixed with a cosolvent system. The cosolvent system includes a first solvent for use in solubilizing said metal organic portion and a thinning agent for use in reducing surface tension in the liquid precursor solution to a value ranging from 10 to 40 dynes per centimeter. This range of surface tension is more preferably from 14 to 34, and is most preferably from 16 to 26. These surface tensions are measured at the ambient temperature in the intended environment of use. This ambient temperature is preferably about 20xc2x0 C. The metal organic portion, the first solvent, and the thinning agent form a substantially homogenous mixture.
Suitable materials for use as the thinning agent or surface tension reducing agent are typically low molecular weight polar hydrocarbons, e.g., tert-butyl alcohol, n-butylineamine, diethylamine, ethyl ether, and isopentane. Methanol is particularly preferred for use with 2-methoxyethanol solvents. Methyl ethyl ketone is particularly preferred for use with xylenes, octane, and other apolar solvents. Inorganic compounds, such as ammonia may also be used. The thinning agent should be nonreactive in solution, but low revel reactivity is permitted if the solution will not be fundamentally altered before it is consumed. It is preferred that the thinning agent have a surface tension in air of 75% or less than the liquid precursor solution to which the thinning agent is added, This requirement means that the thinning agent will usually have a surface tension less than 20 dynes per centimeter at 20xc2x0 C. The thinning agent should also have a boiling point greater than 60xc2x0 C. at atmospheric pressure if it is to be used in misted deposition apparatus. Films of good morphology can be derived from solutions where the thinning agent is present in an amount up to 60% by volume of all of the solvents in the solution.
Mists or aerosols according to the invention are suspensions of liquid particles in a gas. The particles in the mists or aerosols are made of the liquid precursor solution. The particles typically have sizes in the colloidal size range, namely, at least one single dimension in the range from one nanometer to one micron. In this size range, the surface area of the particle is so much greater than its volume that unusual phenomenon occur, i.e., the particles do not settle out of the solution by gravity and are often small enough to pass through filter membranes. Colloidal sizes are not necessarily essential to the formation of an aerosol. It is necessary to deposit these particles on an integrated circuit substrate, e.g., a silicon wafer, to produce a thin film of the liquid precursor solution on the substrate.
A misted deposition apparatus is required for misting of the liquid precursor. Preferred misted deposition apparatus includes ultrasonic or venturi apparatus. The venturi apparatus uses the well-known principles of an automotive carburetor for misting of the liquid. These devices require manual adjustments with empirical measurements of the corresponding particle sizes.
Equation (1) below describes the particle size diameters of particles that are generated by the ultrasonic transducers in an ultrasonic mist generator:                     D        =                              1            2                    *                                                    π                                  F                  a                  2                                            *                              γ                ρ                                      3                                              (        1        )            
wherein D is the particle size diameter, Fa is the frequency of ultrasound coming from the ultrasonic transducers, xcex3 is the surface tension of the liquid precursor solution, and xcfx81 is the density of the liquid precursor solution. The intensity of applied ultrasonic energy does not affect the particle size diameter unless the precursor liquid begins to boil if too much energy is applied and converted to heat energy. The intensity of applied ultrasonic energy affects the volume of mist that is generated, but typically does not affect the particle size diameter.
It is apparent from Equation (1) that particles may be generated which are outside the colloidal range if the surface tension or density of the liquid precursor solution falls outside a range that will yield colloidal particles. The primary concern is for particles that are too large because these particles may wet the substrate by forming pools or beads of liquid that degrade the film quality.
According to Equation (1), the particle size diameter D may be modified by adjusting the surface tension xcex3 or density xcfx81 of the liquid precursor solution. A decrease in the surface tension or an increase in the density results in the formation of smaller particles. Increasing the density of hydrocarbon solutions typically results in an increase in viscosity together with a corresponding increase in surface tension. Thus, density increases are typically overcome or exceeded by the corresponding increase in surface tension. On the other hand, it is possible to reduce particle sizes by adding a surface tension reducing agent. The effects of the thinning agent or surface tension reducing agent may be partially offset by corresponding decrease in solution density.
xcex3=xcex31X1+xcex32X2xe2x88x92xcex2X1X2xe2x80x83xe2x80x83(2)
wherein xcex3 is the altered surface tension of the solution in dynes per centimeter, xcex31 is the surface tension of the first solution in dynes per centimeter, xcex32 is the surface tension of the second solution in dynes per centimeter, X1 is the concentration of the first solution determined as a mole percentage of the total solution, X2 is the concentration of the second solution determined as a mole percentage of the total solution, and xcex2 is an empirically derived temperature dependent constant.
It is also possible to reduce the surface tension by heating the solution. The applied ultrasound is capable of heating the solution. A problem that arises from heating the solution is that too much heating may cause the solution to boil with a corresponding fractional distillation of lighter solvents and surface tension reducing agents. The best surface tension reducing agents according to Equation (2) have relatively low boiling points. The distillation of lighter solution components can be disastrous because these components may condense on the misted deposition apparatus. This condensation may render the apparatus inoperable due to dripping of the condensed materials.
The liquid precursor is used in combination with a misted deposition apparatus having a barrier plate of substantially the same area as the substrate. Here, substantially the same area means that the area of the barrier plate in the plane of the substrate differs from the area of the substrate by only 10% or less. The barrier plate has a smoothness tolerance of 5% of the average distance between said barrier plate and said substrate. The deposited film shows better thickness uniformity than with barrier plates in which the area and tolerances are not within these parameters.
The misted deposition apparatus is used to make integrated circuits and comprises: a deposition chamber; a substrate located within the deposition chamber, the substrate defining a substrate plane; means for producing a mist of a liquid precursor; and means for flowing the mist through the deposition chamber substantially evenly across the substrate to form a film of the liquid precursor on the substrate, wherein the means for flowing includes a barrier plate disposed in spaced relation above the substrate and substantially parallel thereto, the area of the barrier plate in a plane parallel to the substrate being substantially equal to the area of the substrate in the substrate plane. Preferably, the barrier plate has a smoothness tolerance of 5% of the average distance between the barrier plate and the substrate. Preferably, the apparatus further includes means for maintaining the deposition chamber under vacuum, means for applying a DC bias between the barrier plate and the substrate, and means for adjusting the barrier plate to vary the distance between the barrier plate and the substrate. Preferably, the apparatus includes an injection nozzle assembly for injecting the mist into the deposition chamber, in a direction substantially parallel to the substrate plane, disposed in close proximity to and around the periphery of one side of the substrate, an exhaust assembly disposed in close proximity to and around the periphery of an opposite side of the substrate from the injection nozzle assembly, and the substrate, the barrier plate, the injection nozzle assembly, and the exhaust assembly collectively define a semienclosed area within the deposition chamber. Preferably, the apparatus includes means,for rotating the substrate in a plane parallel to the substrate plane while the mist is being deposited on the substrate and means for applying ultraviolet radiation to the mist while the mist is flowing through the deposition chamber. Preferably, the deposition chamber is maintained at substantially ambient temperature while the mist is flowed into it.
Temperature changes are associated with corresponding changes in surface tension and mist particle sizes. The changes can be calculated by known information, e.g., the equation:
xe2x80x83xcex3=xcex1xe2x88x92bT,xe2x80x83xe2x80x83(3)
where xcex3 is the surface tension in dynes per centimeter, T is the temperature in degrees Celsius, and a and b are empirical constants, such as those published in J. J. Jasper, J. Phys. Chem Ref. Data 1, 841 pp. 5.91 to 5.113 (1972), which is hereby incorporated by reference to the same extent as though fully disclosed herein. Ultrasonic misting results in an undesirable heating of the precursor liquid, which may eventually cause the liquid to boil. The temperature of the liquid precursor is preferably kept below a level that results in boiling of the precursor liquid, and this temperature range is usually less than 40xc2x0 C.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description in conjunction with the drawings.